The development of cancer not only is a result of abnormal proliferation, but frequently a loss of normal differentiation. This is particularly the case in Acute Myeloid Leukemia (AML), in which stem cells fail to differentiate into mature myeloid (granulocytic and monocytic) cell types. Therefore, understanding the mechanisms underlying myeloid differentiation is an important step in developing new therapies based on specific disease mechanisms. Myeloid differentiation is a result of activation of the function and/or expression of specific transcription factors, but the mechanism of activation of these transcription factors is not well understood. One critical myeloid transcription factor is the CCAAT Enhancing Binding Protein alpha (C/EBP alpha), which is absolutely necessary for development of mature neutrophils from multipotential progenitors. Recent studies have demonstrated that disruption of its function contributes to the block in differentiation observed in many types of Acute Myeloid Leukemia (AML) and in myeloid blast crisis of Chronic Myelogenous Leukemia (CML). The regulation of C/EBP alpha expression and function is tightly controlled in normal hematopoiesis, and down-regulation of C/EBP alpha expression has been demonstrated in at least one form of AML, t(8;21) M2 characterized by the AMLI/ETO fusion protein. However, the mechanisms by which C/EBP alpha is affected by extracellular signaling by factors such as MAP kinase pathways, as welt as how its expression is regulated in normal myeloid cells and in AML, have not been elucidated. In order to further our understanding of normal differentiation, and how it is disrupted in AML, our specific aims are: (1) To determine the role of signal transduction through C/EBPa in granulopoiesis. (2) To characterize the regulatory regions mediating expression of C/EBP alpha in normal hematopoiesis. (3) To determine the mechanism of AMLI/ETO down regulation of C/EBP alpha in t(8:21) AML. These studies could lead to new strategies for the treatment of myeloid leukemias. This project will interact closely with Project 0005 in the analysis of expression of reporter genes in hematopoietic progenitor subsets in transgenic mouse models (Aim 2); with Project 0003 investigating down-regulation of C/EBP alpha by AMLI/ETO (Aim 3); with Project 0006 determining the significance of activation of C/EBP alpha by signaling pathways (Aim 1); and with Core 9001 in Biostatistical analysis of leukemic models.